Your question can not be answered without making some assumptions so I would have to know more about what you are asking about. For example, are you asking about "life" in the general sense (meaning all living things) or are you asking about the life of a specific organism / individual? Life in the general sense likely began many millions of years ago. Maybe even billions of years ago. We can only make guesses and calculations based about the origins of life in general - based on the fossils and data we have.

The life of an individual organism (a human being for example) has a much more clear and easier to determine 'scientific' beginning and that moment is conception.

Legally, life (lives) begin at conception as well. Conception is recognized as the beginning of life in several laws and scientific disciplines that include laws and regulations on human cloning, patents on human organisms, fetal homicide laws and regulations about IVF (Intravenous fertilization).

I'll leave the moral arguments and the moral aspects for someone else. I don't generally concern myself with those because we all have the first amendment right to 'believe' or to not believe anything along those lines as we wish. .

"Sooner or later, the Supreme Court of the Unites States is going to have explain how a 'child in the womb' is a person enough to be recognized as a MURDER victim under our fetal homicide laws but how they are not persons enough to qualify for any other Constitutional protections" ~ Chuz Life